U.S. patent number 9,595,857 [Application Number 14/437,682] was granted by the patent office on 2017-03-14 for electrical machine.
This patent grant is currently assigned to Openhydro IP Limited. The grantee listed for this patent is Simon Cawthorne, Edward Spooner. Invention is credited to Simon Cawthorne, Edward Spooner.
United States Patent |
9,595,857 |
Cawthorne , et al. |
March 14, 2017 |
Electrical machine
Abstract
An electrical machine comprising a stator and a member arranged
to move relative to said stator. The stator comprises a first
stator structure incorporating a first plurality of windings
disposed along the direction of relative motion and displaced from
the movable member in a direction perpendicular to the direction of
motion, and a second stator structure incorporating a second
plurality of windings displaced in a second direction from the
movable member, so that the movable member lies between the two
stator structures. The movable member is arranged to electrically
couple with the windings. The first plurality of windings includes
a single phase winding connected to a first single-phase ac supply,
and the second plurality of windings includes a single phase
winding of the same pole pitch being displaced in the direction of
relative motion from first plurality of windings and connected to a
second single-phase ac supply whose phase is displaced from that of
the first supply.
Inventors: |
Cawthorne; Simon (Carlingford,
IE), Spooner; Edward (Durham, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Cawthorne; Simon
Spooner; Edward |
Carlingford
Durham |
N/A
N/A |
IE
GB |
|
|
Assignee: |
Openhydro IP Limited
(IE)
|
Family
ID: |
47115564 |
Appl.
No.: |
14/437,682 |
Filed: |
October 29, 2013 |
PCT
Filed: |
October 29, 2013 |
PCT No.: |
PCT/EP2013/072558 |
371(c)(1),(2),(4) Date: |
April 22, 2015 |
PCT
Pub. No.: |
WO2014/067920 |
PCT
Pub. Date: |
May 08, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150295482 A1 |
Oct 15, 2015 |
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Foreign Application Priority Data
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|
|
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Oct 31, 2012 [EP] |
|
|
12190733 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K
16/04 (20130101); H02K 7/1823 (20130101); H02K
41/02 (20130101); H02K 3/48 (20130101); H02K
1/165 (20130101); H02K 1/16 (20130101); H02K
2203/15 (20130101) |
Current International
Class: |
F03B
13/00 (20060101); H02K 16/04 (20060101); H02K
1/16 (20060101); H02K 7/18 (20060101); H02K
3/48 (20060101); H02K 41/02 (20060101) |
Field of
Search: |
;290/43,54
;310/112,114,179,210 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
2462257 |
|
Feb 2010 |
|
GB |
|
2488129 |
|
Aug 2012 |
|
GB |
|
2005117243 |
|
Dec 2005 |
|
WO |
|
2012059109 |
|
May 2012 |
|
WO |
|
Other References
International Search Report for PCT/EP2013/072558, mailed Feb. 18,
2014, pp. 1-3. cited by applicant.
|
Primary Examiner: Gonzalez; Julio C.
Attorney, Agent or Firm: Tarolli, Sundheim, Covell &
Tummino LLP
Claims
The invention claimed is:
1. An electrical machine comprising a stator and a member arranged
to move relative to said stator in a direction of relative motion,
the stator comprising a first stator structure incorporating a
first plurality of windings disposed along the direction of
relative motion and displaced from the movable member in a
direction perpendicular to the direction of relative motion, and a
second stator structure incorporating a second plurality of
windings displaced in a second direction from the movable member,
so that the movable member lies between the two stator structures,
the movable member being arranged to electrically couple with the
windings, wherein the first plurality of windings includes a single
phase winding having a pole pitch, the second plurality of windings
includes a single phase winding of the same pole pitch being
displaced in said direction of relative motion from first plurality
of windings, wherein the displacement of the second plurality of
windings with respect to the first is half of one pole pitch in
said direction of relative motion, and wherein each stator
structure comprises slotted laminations having a respective group
of slots for each pole of the respective single phase winding, and
wherein the groups of said first stator structure are displaced in
the direction of relative motion from the groups of said second
stator structure.
2. The electrical machine of claim 1 wherein the plurality of
windings of each stator structure is split into sections with gaps
between the sections arranged so that the travelling magnetic field
within the machine induces single phase alternating emf in the
sections displaced in phase by 120 electrical degrees, the sections
of winding being connected to one of: a three-phase ac-dc power
converter or voltage source.
3. The electrical machine of claim 2 wherein the windings of the
first stator structure have more turns than those of the second
stator structure by a factor of approximately square root of 3, the
windings of the first stator structure being connected in delta to
a three-phase power converter or voltage source and the windings of
the second stator structure being connected in star to the same
three-phase converter or voltage source.
4. The electrical machine of claim 1, wherein said machine
comprises a rotary machine in which the stator and the movable
member are curved about an axis of rotation, the movable member
comprising a rotor.
5. The electrical machine of claim 4, wherein the stator structures
are displaced axially from the rotor and the magnetic field in each
region between the rotor and a stator structure passes in a
substantially axial direction.
6. The electrical machine of claim 5 wherein the stator structures
are formed from sets of arc segments arranged to form at least a
partially circular array about the axis of rotation.
7. The electrical machine of claim 5 wherein the stator structures
are formed from sets of substantially rectangular blocks arranged
to locate tangentially to form at least a partially circular array
about the axis of rotation.
8. The electrical machine of claim 5 wherein the plurality of
windings of each stator structure is split into sections with gaps
between the sections arranged so that the travelling magnetic field
within the machine induces single phase alternating emf in the
sections displaced in phase by 120 electrical degrees, the sections
of winding being connected to one of: a three-phase ac-dc power
converter or voltage source.
9. The electrical machine of claim 4, wherein the stator structures
are formed from sets of arc segments arranged to form at least a
partially circular array about the axis of rotation.
10. The electrical machine of claim 4, wherein the machine
comprises one of a permanent magnet synchronous machine or as a
brushless-dc machine wherein the rotor comprises a cylindrical
array of permanent magnets.
11. The electrical machine of claim 1, wherein the machine
comprises an induction machine in which said movable member
comprises a sheet of electrically conductive material.
12. The electrical machine of claim 1, wherein the machine
comprises one of a permanent magnet synchronous machine or a
brushless-dc machine in which said movable member comprises an
array of permanent magnets.
13. The electrical machine of claim 1 wherein each stator structure
comprises slotted laminations, housing groups of concentric coils
of electrical conductor.
14. The electrical machine of claim 13, wherein each group of coils
within each single phase winding is formed by a continuous
insulated cable.
15. The electrical machine of claim 1 wherein the respective slots
of each of said first and second stator structure are non-uniformly
spaced apart with the spacing between successive slots being
progressively increased and then progressively decreased to create
an approximately sinusoidal distribution of said slots.
16. The electrical machine of claim 1, wherein the spacing between
adjacent slots of each group of slots becomes progressively smaller
in a direction towards a centre of the respective group.
17. The electrical machine of claim 1, wherein the respective slots
of each of said first and second stator structures are uniformly
spaced apart with successive slots accommodating a progressively
higher number of turns of the respective first or second winding
and then progressively lower number of turns of the respective
first or second winding to create an approximately sinusoidal
distribution of turns in said slots.
Description
RELATED APPLICATIONS
The present invention is a U.S. National Stage under 35 USC 371
patent application, claiming priority to Serial No.
PCT/EP2013/072558, filed on 29 Oct. 2013; which claims priority
from European Application No. 12190733.1, filed 31 Oct. 2012, the
entirety of both of which are incorporated herein by reference.
FIELD
The present invention relates to an electrical machine.
BACKGROUND
In many types of electrical machine, electric currents in a stator
create a travelling magnetic field that interacts with a set of
electric currents, a set of permanent magnets or a set of
ferromagnetic features on the moving part of the machine, i.e. a
rotor in the case of a rotary machine, or a translator in the case
of a linear machine. The usual method of creating the travelling
field is to use three component windings housed together in a
uniform array of slots in a laminated iron stator and fed with
three alternating currents from a three-phase electrical power
supply. Each component winding has a cyclic distribution of coils
and the three component windings are placed with their magnetic
axes spaced at intervals of 1/3 of a wavelength along the stator.
The alternating currents fed to the windings have relative phase
differences of 120 degrees. As a result, the combined magnetic flux
approximates a sinewave of constant amplitude travelling at a speed
equal to one wavelength of the cyclic distribution during one cycle
of the alternating current. Other polyphase windings are possible
but are seldom used because the prevalent form of power supply is
the three-phase type.
The most common type of electrical drive, the three-phase induction
motor of the type used in many industrial applications, employs
this method to produce a rotating field. Referring to FIG. 1(a), a
rotor 12 carries a set of conducting bars (not shown) in which
electric currents are induced and which interact with the rotating
field to produce torque on an output shaft.
In domestic and light industrial applications, where a three-phase
electrical supply is not available, an alternative arrangement is
used whereby two component windings are supplied with alternating
currents differing in phase. The first current is provided directly
by an available single-phase supply and the second is obtained from
the same supply usually via a capacitor that introduces a phase
shift. The phase shift is of the correct degree only under one load
condition so under most conditions the operation of such motors is
not ideal. Such machines are referred to as single-phase motors
because they operate from a single-phase supply but the windings
are more accurately described as two-phase windings. If a balanced
two-phase supply providing two currents displaced in phase by 90
degrees were available, electrical machines of this type with
component windings displaced by 1/4 of the wavelength would create
a rotating field of constant amplitude and could be just as
effective as standard three-phase machines.
In recent years, linear electric motors have attracted interest for
several applications including guided ground transport and for
electromagnetic launch systems on aircraft carriers. Also linear
generators have been used in certain wave-power devices relying on
reciprocating movement. A linear electrical machine can be
considered as a standard rotary machine that has been cut and
un-rolled, as shown in FIG. 1(a) . . . (c). When this is done, the
magnetic force of attraction indicated by the force lines 16
between stator 10 and translator 14 is no longer balanced by equal
and opposite forces as in the case of the stator 10 and rotor
12.
A common approach to overcome this problem is to use two stators
10', 10'' placed on opposite sides of a single translator 14 as
shown in FIG. 2. In most cases magnetic flux 18 passes from the
first stator 10', through the translator 14, through the second
stator 10'', and through the translator for a second time to
complete its circuit.
The two-stator linear machine could be rolled up around the
original axis (shown as A in FIG. 1(b)) to form a rotary machine
with two co-axial stators enclosing a hollow cylindrical rotor and
with magnetic flux passing radially from the inner stator, through
the inner gap, the cylindrical rotor and the outer gap to the outer
stator. Machines of this configuration have applications in servo
control systems where a drive of very low inertia is required.
Alternatively, the linear machine may be rolled up around an axis
orthogonal to the original axis (shown as B in FIG. 2) to create a
rotary machine where the flux passes axially across two planar
airgaps rather than radially through cylindrical airgaps.
Axial-flux rotating machines of this type have been used
particularly as permanent-magnet generators for renewable energy
applications, notably small wind turbines. In such cases each of
the two stators normally carries a three-phase winding comprising
three component windings as described above.
It is an object of the present invention to provide an improved
electrical machine.
SUMMARY
According to the present invention there is provided an electrical
machine as claimed in claim 1.
In embodiments of the invention, a two-stator machine has a
two-phase winding in which one component winding is housed in each
stator and the two component windings are aligned with their
magnetic axes displaced by approximately 90 electrical degrees.
The displacement between the magnetic axes of the two windings is
ideally 90.degree. and the windings are ideally supplied with
alternating currents that differ in phase by 90.degree.. The
invention applies to motors or generators in linear or either
radial- or axial-flux rotary form. Each stator carries a single
winding and this removes the restrictions caused by each winding
having to share a common array of slots. As a result: 1. The
winding distribution can be arranged to approximate closely to a
sinusoidal pattern because the slots no longer need to be uniformly
distributed and can be distributed according to the requirements of
just the one winding housed in them. 2. The winding layout is
simplified because the winding does not need to overlap other
windings and so it may be a straightforward set of planar coils in
the case of a linear machine or an axial-flux rotary machine 3. The
insulation is simpler because the winding needs only to be isolated
from ground and not from other windings. 4. As a consequence of 2
and 3 it becomes practicable to form each winding from a continuous
length of insulated cable. For undersea applications, this is
especially beneficial as it simplifies the waterproofing the stator
electrics as all winding connections can be readily made within a
single sealed unit.
The movable member, whether a rotor or translator, may comprise a
conducting body so that the machine operates as an induction motor
or generator. Such an induction generator is tolerant to
hypocycloidal motion produced by movement of a shaftless rotor
within a turbine of the type shown in FIGS. 3-5. The rotor or
translator may also include ferromagnetic parts to assist the
passage of magnetic flux between the two stator structures.
Alternatively, the rotor or translator may comprise an array of
permanent magnets or electromagnets so that the machine operates as
a synchronous machine. With appropriate control of the electrical
feed, such a machine would form a so-called brushless-dc
machine.
A further alternative is to use a rotor or translator that includes
an array of ferromagnetic features that interact with the magnetic
field of the stator to create torque in the case of a rotary
machine or thrust in the case of a linear machine. With appropriate
control of the electrical feed, such a machine could form a
synchronous reluctance machine, a switched-reluctance machine or a
stepper motor.
The two windings may be fed from a two-phase electrical source or,
in the case of a generator, they may feed into a two-phase load. A
two-phase source may be provided by: 1. a special-purpose two-phase
alternator; 2. a power-electronic converter with two-phase output,
such as may be arranged using two H-bridges using IGBTs and
connected to a common dc link; 3. a three-phase source with a set
of transformers connected to convert between three-phase and
two-phase systems, such as a Scott-T connected transformer; or 4. a
single-phase source with a phase-shifting impedance as used in a
single-phase motor.
A two-phase load for a generator may employ one of the above
arrangements operating with reversed power flow or it may comprise
simply resistive load banks. In the case of a long linear machine
or a large-diameter rotary machine, it is preferred to divide the
stators into a number of manageable sections. For a rotary machine,
the sections may be a set of arcs disposed around the machine or
alternatively if a large number of sections is used, then in the
case of an axial-flux machine it may be convenient and more cost
effective to make the sections straight and disposed tangentially.
If the sections are spaced apart, then the emfs in their windings
differ in phase. If the distance between sections is 1/3 of a
wavelength then the phase difference is 120 degrees and if the
stator has been divided by a multiple of three, then a three-phase
supply can be used to supply the sections of one stator. If the
distance is 1/6 of a wavelength then a three-phase source may be
used with the windings of alternate blocks reversed. In either
case, a second three-phase supply with its three outputs in phase
quadrature with those of the first three phase supply can be used
to supply the sections of the second stator. If the windings of the
sections of the first stator are connected in star, then the
required second three-phase supply can be provided by connecting
the windings of the second stator in delta and to the same
three-phase source. In this way a standard three-phase source can
be used. It is noted that the voltage applied to the windings
connected in delta is higher than the voltage applied to the
windings connected in star and the current is less. The windings of
the two stators are therefore preferably arranged to operate at
different voltage and current by specifying proportionately
different numbers of turns of conductor of different cross
section.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
FIGS. 1(a) to (c) show schematically the components of a rotary and
a linear electrical machine;
FIG. 2 shows a two-stator linear machine;
FIG. 3 shows in elevation a housing for an open centered
hydro-turbine including an electrical generator according to an
embodiment of the present invention;
FIG. 4 shows a perspective view of the housing of FIG. 3;
FIG. 5 is a cross-sectional view through the line V-V and the plane
V-V indicated in FIGS. 3 and 4;
FIG. 6 shows an exemplary layout of windings on a stator for the
turbine of FIGS. 3-5;
FIGS. 7(a) and (b) show in detail the layout of windings for a pole
of the stator of FIG. 6;
FIG. 8 shows a cross-section of an exemplary laminate for
accommodating the windings of FIGS. 6 and 7;
FIG. 9 shows a cross-section of an alternative laminate for
accommodating windings for a stator;
FIG. 10 shows an electrical connection for the windings of the two
stators arranged to operate with a standard three-phase electrical
system;
FIG. 11 shows a two-bridge converter for use with the system of
FIG. 10; and
FIG. 12 shows a converter providing a two-phase output for use in
alternative implementations of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One particular application for the present invention is an
axial-flux, low-speed, direct-coupled, rotary generator for use in
an open-centre, shaftless, tidal-current turbine. Referring now to
FIGS. 3 and 4, there is shown such a turbine 30 including an
electrical motor/generator according to an embodiment of the
present invention. The turbine comprises a runner, comprising a set
of blades 32 that are fixed between an inner ring 34 and an outer
ring 36, housed in a shroud 38. Typically, the turbine housing also
includes mounts which allow the turbine to be secured to a subsea
structure such as a gravity base--although this detail is not shown
in the present case.
Turning now to FIG. 5 which shows a portion of the turbine in
cross-section. The outer ring 36 runs within a channel defined by a
front annular support 40, an outer cylindrical support 42 and a
rear annular support 44--front and rear being defined relative to
the movement of fluid through the turbine indicated by the arrow F.
Nonetheless, it will be appreciated that the turbine can be
bidirectional and so responsive to fluid flow in either direction.
Each support 40, 42, 44 is separated from the ring 36 by a
respective bearing 40', 42', 44'. In the embodiment, none of these
components need be magnetic and can be designed purely from a
mechanical transmission efficiency point of view.
In this embodiment, a motor/generator assembly is disposed axially
away from the turbine assembly. The motor/generator assembly again
comprises a front annular support 46, a cylindrical outer support
48 and a rear annular support 50. The front annular support 46 can
be fixed to the rear annular support 44 or indeed these can be
produced as an integral component. A first circular stator
comprises a set of coils 52 wound around a circular laminate 53 as
described in more detail below and the laminate 53 is fixed to the
front support 46. A second circular stator comprises a set of coils
54 wound around a circular laminate 55 again as described in more
detail below and the laminate 55 is fixed to the rear support 50.
An annular rotor 56 is disposed between the two stators 52, 54 and
this motor/generator rotor 56 is directly coupled to the outer ring
36 of the runner via a transmission ring 60. Thus, when the
motor/generator 52, 54, 56 is driven to start the turbine, rotation
is transmitted to the runner via the ring 60, and once running, the
runner drives the motor/generator via the ring 60. It will
nonetheless be appreciated that in the right tidal conditions,
tidal flow could be used to start the turbine without the need to
kick-start the turbine electrically. However, the possibility of
kick-starting the turbine means that it can operate across a
greater range of conditions including low flow conditions than if
this option were not available.
FIG. 6 shows the construction of the stators 52, 54 of FIG. 5 in
more detail. In some applications, the stator could have a mean
diameter of 9 m and in this case a pole number of 48, i.e. the
winding 53, 55 on each stator 52, 54 is a single-phase 48-pole
arrangement. While it will be appreciated that the second stator 54
is identical to and facing the first 52, in a 48 pole arrangement
it is circumferentially displaced from the first by an angle of
3.75.degree. which is half of a pole pitch.
In one embodiment, the winding of each stator is formed from 12
lengths of insulated cable laid into 864 slots. FIG. 7a illustrates
the first stage 60(1) of the cable layout for one pole 60, where
the number of slots per pole has been reduced from 18 to 10 in
order to simplify the illustration. Cable laying progresses in a
helical manner from the outside to the inside to form a
single-layer, 9-turn, planar coil. Cable laying then proceeds as
shown dotted in FIG. 7(a) and a second layer 60(2) is formed in the
same slots in the pattern shown in FIG. 7(b), which finishes at the
outside. On completion of the second layer, cable laying may
proceed to form two further layers in the same slots.
Alternatively, cable laying may proceed to form two layers on an
adjacent pole. In the present example, each of the 12 lengths of
cable is used to form a total of 16 pairs of layers and these may
be arranged as 2 layers on each of 16 poles, 4 layers on each of 8
poles or 8 layers on each of 4 poles. It will be seen that the two
ends of each length of cable are situated on the outside of the
windings and so it is convenient to provide sufficient spare length
to reach an electrical equipment enclosure (not shown) where the
cable ends may be passed through watertight glands. This
arrangement is favourable for subsea use as it avoids any cable
joints or splices that are expensive to form and are vulnerable
points where electrical faults could occur. The 12 lengths of cable
may be connected in parallel to a common single-phase ac source or
they may be isolated and connected to separate independent
single-phase sources so that in the event of a cable failure or
failure of a source, the system can continue to operate.
Each of the stator cores 53, 55 is ideally a continuous laminated
ring that could be formed from a continuous spiral of electrical
steel sheet. However a continuous ring of the required dimensions
could be unwieldy and in some applications, it is preferred to
divide the core into a number of arcs or short straight lengths
that assemble to form a polygon. In the present example, the
preferred arrangement has 12 arcs or straight lengths and the cable
winding is formed as four poles of eight layers on each. The
winding may therefore be formed before the core sections are
assembled to form the complete stator ring (or polygon).
In one embodiment, slots 80 are distributed as shown in FIG. 8 with
positions selected to give progressively higher concentrations of
turns further from the pole axes, giving a good approximation to a
sinusoidal current distribution. FIG. 8 shows sections from the two
stators facing each other and displaced by a half pole pitch.
In alternative arrangements, the slots can be arranged in a uniform
array of slots but with each slot containing different numbers of
turns. This can be achieved by omitting some of the inner loops of
cable around each pole for the first few layers. In this case, the
slot depth may differ according to the number of turns allocated or
the slots may be all of the same depth and those containing fewer
turns may contain inert packing. The illustration in FIG. 9 shows a
lamination with deep slots 90 furthest from the pole axis which
could contain 8 layers of cable. The first two layers of the
winding for each pole would have 2 loops. Closer to the pole axis,
shallower slots allow the next two layers to have 3 loops and the
last four layers contain 4 loops.
A combination of the approach shown in FIGS. 8 and 9 for achieving
a sinusoidal distribution could also be adopted. The benefit of
achieving a close approximation to a sine wave is the reduction in
losses associated with space harmonic components of the current
distribution. Harmonic components give rise to flux components that
rotate at speeds different from the speed of the fundamental and
induce unwanted currents within the rotor leading to additional
losses.
FIG. 10 is an electrical diagram showing an exemplary electrical
system for the generator. In one implementation, three windings a,
b and c are connected in star to the source/grid 100 and are housed
with one stator; whereas a second set of windings A, B and C are
connected in delta to the source/grid and housed with the other
stator. Each stator structure is split into sections with the
sections arranged so that the travelling magnetic field within the
machine induces single phase alternating emf in the sections
displaced in phase by 120 electrical degrees. Thus, each winding a,
b, c, A, B, C is connected to a respective section of each stator.
The delta-connected windings A, B, C should have approximately 3
times as many turns as those in star connection because they
operate with the same magnetic flux but at 3 times higher voltage.
However they carry only 1/ 3 as much current to deliver the same
power and so their windings may be of correspondingly smaller cross
section.
Exemplary power conversion circuitry 100 comprising a
voltage-source inverter is described in PCT/EP2012/065701. FIG. 11
in particular shows a two-bridge converter where both a first stage
converter 110 and a second stage converter 120 comprise a 3-phase
voltage source inverter connected via a DC link 140. The phase
ports of the first stage 110 are connected to the source/grid,
whereas the phase ports P1, P2, P3 of the second stage 120 are
connected to the stator blocks as shown in FIG. 10.
Referring now to FIG. 12, in other implementations, each stator's
single phase winding can be connected to a power converter 110,
140, 125 comprising a respective H bridge that converts a dc source
140 to a single-phase ac source, phase A and phase B, the dc ports
of the two H bridges 125 being connected to a common dc source 140
and their ac ports being arranged to provide voltages phase A and
phase B, that differ in phase.
The rotor 56 is an annular sheet or plate of electrical conductor
that rotates in the gap between the two stators as illustrated in
FIG. 5. The annulus may be fabricated from several arcs or
trapezoidal sheets. It is best to avoid using a number of sheets
equal to the number of stator sections so that any torque
pulsations created by the joints tend to mutually cancel.
* * * * *